The present invention relates to multimedia terminals and especially a multimedia terminal comprising a first encoder for encoding a first signal for producing a first bit-stream of a first media type and having a first bit-rate, a second encoder for encoding a second signal for producing a second bit-stream of a second media type and having a second bit-rate, and a multiplexer for combining at least the first and the second bit-streams into a third bit-stream.
In multimedia transmission, separately encoded bit-streams from a sender""s different media sources (e.g. video, audio, data and control) are multiplexed into a single bit-stream, and at the receiving end the bit-stream is again de-multiplexed into various multimedia streams to be decoded appropriately. The block diagram of FIG. 1 illustrates the principle of multiplexing by a prior art solution for combination of encoded speech and video data streams in a videophone terminal. The terminal comprises a video encoder 100 and a speech encoder 110. A speech input signal 114 and a video input signal 106 are fed to corresponding separate encoders where they are processed with encoding algorithms. The resulting encoded bit-streams 112, 107 are fed to relevant bit-stream buffers 111, 101 of the encoders. The bit-stream from the video bit-stream buffer 105 and the bit-stream from the speech bit-stream buffer 113 are input to a multiplexer 120, which combines the separate bit-streams into a composite bit-stream 123 that is forwarded to the transmission means of the multimedia terminal.
Even though the coding algorithms effectively compress data, the limiting factor of the process, especially in terminals that operate over a radio interface, is transmission capacity, and therefore optimization of the use of this limited resource is very important. In videophone solutions the bit-rate of the video encoder output stream is typically controllable, and this has been used to divide the limited transmission resources between the different media types in the multiplexed data flow.
Document ITU Telecommunication Sector, Video Codec Test Model, Near-Term Version 8 (TMN8), Document Q15-A-59, Portland, Jun. 24-27, 1997 describes a typical prior art videophone application, where the constant bit-stream of the speech encoder is first defined, after which the variable-rate video encoder output stream is adapted to the remaining capacity by adjusting the spatial resolution of the video coding. If the predefined targets are met, the video encoder produces a bit-stream with a constant bit-rate. For situations where at least one (e.g. speech coding) or even more (e.g. signaling) functions are implemented with variable bit-rate, this adjustment scheme is too rigid and the available transmission capability is not optimally utilized, since space may be left unused in the multiplexer buffer.
Furthermore, the use and importance of different media sources vary very much according to the purpose and environment of the connection. Conventionally voice has been given a clear preference over other types of media. When terminals improve and their usage diversifies, preferences in different situations will also change. In some cases voice will be preferred over video, but in other cases good quality transmission of video may be considered more important. Sometimes a good compromise between the two, adjusted to the transmission conditions, would be appreciated. Accordingly, in addition to the inherent need for optimising the use of transmission capacity of a multiplexed multimedia data stream, a need has risen for adjusting the trade-off between different data streams according to the purpose and situation of the user or the condition of transmission link in use.
Now a multimedia terminal and a method for use in a multimedia terminal have been invented by use of which the presented disadvantages can be reduced and a possibility for meeting the new objectives is enhanced. According to a first aspect of the present invention there is provided a multimedia terminal a first encoder for encoding a first signal for producing a first bit-stream of a first media type and having a first bit-rate, a second encoder for encoding a second signal for producing a second bit-stream of a second media type and having a second bit-rate, a multiplexer for combining at least the first and the second bit-streams into a third bit-stream. The terminal is characterized by comprising an input element for receiving preference information coupled to the first encoder and the second encoder, said preference information indicating a preferred combination of the first and the second media types in the third bit-stream and affecting the first and the second bit-rates.
In the present invention target bit-rates are interactively defined and controlled by control information that affects the encoding function of different encoders. The terminal is provided with means for receiving information that indicates a preference between different media types in the multiplexed bit-stream. The received preference information is used as control information in the encoding processes. Consequently, the transmission capacity is utilised in a more optimised manner and the proportions of different media types are better adjusted to the purpose of the information transfer.
According to a second aspect of the present invention there is provided a protocol for communicating between a first multimedia terminal and a second multimedia terminal, said first multimedia terminal comprising a first encoder for encoding a first signal for producing a first bit-stream of a first media type and having a first bit-rate; a second encoder for encoding a second signal for producing a second bit-stream of a second media type and having a second bit-rate; a multiplexer for combining the first and the second bit-streams into a third bit-stream, said protocol comprising formatted signals for transferring information between the first and the second multimedia terminal. The protocol is characterised by comprising a message for indicating the capability of the first multimedia terminal to control the first and the second bit-rates according to a preference information received by first multimedia terminal, said preference information indicating a preferred combination of the first and the second media types in the third bit-stream and affecting the first and the second bit-rates.
According to a third aspect of the present invention there is provided a method for controlling multiplexing of a multimedia transmission comprising: encoding a first signal for producing a first bit-stream of a first media type and having a first bit-rate; encoding a second signal for producing a second bit-stream of a second media type and having a second bit-rate; combining at least the first and the second bit-streams into a third bit-stream. The method is characterised by receiving preference information, said preference information indicating a preferred combination of the first and the second media types in the third bit-stream; and adjusting the first and the second bit-rates according to the received preference information.